1. Field of the Invention
The present invention relates to photographic elements and processes. More particularly, this invention relates to the art of electrophotography and to an element and process for producing positive images.
2. Description of the Prior Art
Photoconductors have been used in a variety of ways to produce photographic images. In one type of process, an activated photoconductor is used to alter an image-forming material. In another type of process, the photoconductor is used to create an imagewise electric field or imagewise current which can be used to form an image.
In U.S. Pat. No. 3,784,375 to Gilman and Kaukeinen, there is described a process wherein an electrostatic charge pattern exposes silver halide to produce a silver metal latent image. The electrostatic charge pattern is formed by imagewise exposing a uniformly charged photoconductive layer. The photoconductive layer is then brought into contact with a layer containing silver halide. The silver halide layer is then conventionally processed to form a negative visible image.
In U.S. Pat. No. 3,660,087 to Kaspaul, there is described a process wherein an activated photoconductor produces nucleation sites in a material comprising an actinic radiation sensitive material such as zinc oxide and a metallic compound such as cuprous oxide. The resulting latent image is developed to a negative visible image by contacting the element with a vapor of the imaging material. A similar process is described in British Pat. No. 1,314,238.
If a photoconductor is placed in an electric field and then imagewise exposed, an imagewise pattern of electric field is produced. The imagewise pattern of electric field will, of course, produce an imagewise current provided the materials in the field are sufficiently conductive. The imagewise electric current or the field can be used to produce a negative image in a suitable material. For example, the current can be used to expose a silver salt as is described in copending application Ser. No. 624,815, filed Oct. 22, 1975; or the field can be used to enhance the sensitivity of a suitable material such as described in U.S. Pat. No. 3,316,088 to Schaffert. In this latter patent, a decomposable reactive component is simultaneously subjected to imagewise light exposure and an imagewise electric field. The imagewise electric field enhances the photosensitivity of the reactive component. A typical reactive component according to the teaching of this patent is silver azide.
In U.S. Pat. No. 3,898,458 to Reithel, there is described a process where a pigment such as titanium dioxide is activated by an imagewise current flow formed by exposing an inorganic photoconductor to X-rays. The pigment and its support and the photoconductor and its support form a composite element at the time of exposure. The activated pigment is then physically developed. The activated pigment is not itself catalytic but reduces heavy metal salts from the physical developer solution to form metal images which are catalytic. In some embodiments, the activated pigment must be contacted with a separate nucleating agent in order to produce catalytic sites for physical development. The nucleating agent is typically a simple solution of silver nitrate. A positive image can be formed using this process by initially overall activating the pigment either by light exposure or uniform charging. Depending on the polarity of the photoconductor during a subsequent exposure of the composite element, the pigment can be imagewise inactivated.
In another type of process, an electric field is used to transport a reactant and thereby form an image. A typical example of a process of this type is described in U.S. Pat. No. 3,457,069 to Robillard. According to the process of this patent, ions are transported into a semiconductive layer by an electric field. The ions initiate a chain reaction which forms a colored image in the semiconductor layer. The ions may be formed from a mixture of a metal salt, such as silver halide, with a metal oxide such as cuprous oxide. A positive image can be formed when photoelectrons from a light source imagewise neutralize metal ions. In the image areas, there are no metal ions to be transported by the electric field and a positive image results.
Since photoconductive imaging has a very high potential for resolution, there is a continuing need for inexpensive and simple elements and processes for use in photoconductive imaging.